At the present time, the vast majority of RF communication receivers are of the superheterodyne type. This type of receiver uses one or more IF (intermediate frequency) stages for filtering and amplifying signals at a fixed frequency within an IF chain. This radio architecture has the advantage that fixed filters may be used in the local oscillator (LO) chain. In order for the receiver to be useable over multiple bands, its typical architecture is implemented as the single-band receiver shown in FIG. 1. An RF signal arriving at an antenna 11 passes through a band-select RF filter 13, a low noise amplifier (LNA) 15, and into an image filter 17, which produces a band-limited RF signal. This band-limited RF signal then enters the first mixer 19, which translates the RF signal down to an intermediate frequency by mixing it with the signal produced by the first LO 21. The undesired mixer products in the IF signal are rejected by an IF filter 23. The filtered IF signal then enters an IF amplifier stage 25, after which the outputs feeds into the second mixer 27 that translates it down to yet another intermediate frequency by mixing it with the signal produced by a second LO 28. The signal is then sent to the baseband for processing. Tuning into a particular channel within the band-limited RF signal is accomplished by varying the frequency of each LO 21 and 28.
In order to reduce size, power consumption, and cost, it may be advantageous to integrate the electronic components of radio receiver and transmitter to reduce the number of filters and mixers. The superheterodyne design, however, requires high quality, narrowband IF bandpass filters that are typically implemented off-chip. These filtering components impose a lower limit to the size, materials cost, assembly cost, and power consumption of receivers and transmitters that are built using the superheterodyne design. Moreover, the necessity for mixer and local oscillator circuits operating at high frequencies contributes greatly to the power consumption and general complexity of the superheterodyne receiver. In particular, the high-frequency analog mixers require a large amount of power to maintain linear operation. Although many variations of the superheterodyne design exist, they all share the limitations of the particular design just described.
There may be attempts to design radio receivers that permit the integration of more components onto a single chip because of the growing demand for portable communications. Recent advances in semiconductor processing of inductors can allow more and more of these filters to be implemented on-chip.
A second receiver design is the direct-conversion, or zero-IF, receiver shown in FIG. 2. An antenna 57 couples a RF signal through a first bandpass RF filter 59 into an LNA 61. The signal then proceeds through a second RF filter 63, yielding a band-limited RF signal, which then enters a mixer 65 and mixes with an LO frequency produced by an LO 67. Up to this point, the direct-conversion receiver design is essentially the same as the previous receiver design. Unlike the previous designs, however, the LO frequency is set to the carrier frequency of the RF channel of interest. The resulting mixer product is a zero-frequency IF signal—a modulated signal at baseband frequency. The mixer output 67 is coupled into a lowpass analog filter 69 before proceeding into baseband information signal for use by the remainder of the communications system. In either case, tuning can be accomplished by varying the frequency of LO 67, thereby converting different RF channels to zero-frequency IF signals.
Because the direct-conversion receiver design can produce a zero-frequency IF signal, its filter requirements are greatly simplified—no external IF filter components are needed since the zero-IF signal is an audio frequency signal that can be filtered by a low-quality lowpass filter. This allows the receiver to be integrated in a standard silicon process from the mixer 65 stage onwards, making the direct-conversion receiver design potentially attractive for portable applications.
The direct-conversion design, however, has several problems, some of which are quite serious. As with the other designs described above, the RF and image filters required in the direct-conversion design must be high-quality narrowband filters that must remain off-chip. Moreover, this design requires the use of high-frequency mixer and LO circuits that require large amounts of power. Additionally, radiated power from LO 67 can couple into antenna 57, producing a DC offset at the output of mixer 65. This DC offset can be much greater than the desired zero-IF signal, making signal reception difficult. Radiated power from LO 67 can also affect other nearby direct-conversion receivers tuned to the same radio frequency.
The active subharmonic mixer can be a circuit to reduce the local oscillator self-mixing and radiation problems in a direct conversion (or low IF) receiver by using multiple phases of a subharmonic frequency in multi-stack double-balanced active mixer topology. FIG. 3 is block diagram of a conventional subharmonic mixer. In this mixer, RF inputs 71 and 72 are converted to currents by transistors 79 and 80. The in-phase local oscillator signals 73 and 74 drive the first stage of current commutators of transistors 81-84, and the quadrature local oscillator signals 75 and 76 drive the second stage of current commutators 85-88. The resulting currents are converted to output voltages 77 and 78 by resistors 89 and 90. These techniques rely on active mixers that do not scale well with lower supply voltages, have significant non-linearity, have high power dissipation, and can not be effectively implemented in MOS technologies.